The Field Reversed Configuration (FRC) belongs to the class of magnetic plasma confinement topologies known as compact toroids (CT). It exhibits predominantly poloidal magnetic fields and possesses zero or small self-generated toroidal fields (see M. Tuszewski, Nucl. Fusion 28, 2033 (1988)). The traditional method of forming an FRC uses the field-reversed θ-pinch technology, producing hot, high-density plasmas (see A. L. Hoffman et al., Nucl. Fusion 33, 27 (1993)). A variation on this is the translation-trapping method in which the plasma created in a theta-pinch “source” is more-or-less immediately ejected out one end into a confinement chamber. The translating plasmoid is then trapped between two strong mirrors at the ends of the chamber (see, for instance, H. Himura et al., Phys. Plasmas 2, 191 (1995)).
Significant progress has been made in the last decade developing other FRC formation methods: merging spheromaks with oppositely-directed helicities (see, e.g. Y. Ono et al., Nucl. Fusion 39, 2001 (1999)) and by driving current with rotating magnetic fields (RMF) (see, e.g. I. R. Jones, Phys. Plasmas 6, 1950 (1999)) which also provides additional stability. Recently, the collision-merging technique, proposed long ago (see, e.g. D. R. Wells, Phys. Fluids 9, 1010 (1966)) has been significantly developed further: two separate theta-pinches at opposite ends of a confinement chamber simultaneously generate two plasmoids and accelerate the plasmoids toward each other at high speed; they then collide at the center of the confinement chamber and merge to form a compound FRC. In the construction and successful operation of one of the largest FRC experiments to date, the conventional collision-merging method was shown to produce stable, long-lived, high-flux, high temperature FRCs (see, e.g. M. Binderbauer et al., Phys. Rev. Lett. 105, 045003 (2010)).
When an FRC translates into the confinement section, it induces eddy currents in any conducting structure within its vicinity (e.g. the vessel wall or conducting in-vessel components). These eddy currents influence the plasma state and decay over time, thereby contributing to a continuous evolution of the plasma and preventing any steady-state until the eddy currents have decayed to negligible magnitudes. If the conducting structures are not axisymmetric (which is generally the case), the eddy currents break the axisymmetry of the FRC. Overall, such translation-induced eddy currents are undesirable. Their initial excitation imposes constraints on the plasma shape and thereby limits the ability of conducting structures to provide passive stabilization of plasma instabilities, and their decay over time complicates plasma control by requiring continuous compensation even in the absence of plasma instabilities. Furthermore, any beneficial effects of translation-induced eddy currents can also be provided by suitable adjustments of the equilibrium magnetic field.
Translation-induced eddy currents are not the only type of eddy currents that arise during experiments. Plasma instabilities may excite eddy currents which reduce the growth rate of the instability and are thus desirable. Eddy currents will also arise in response to neutral beam current ramp-up.
Plasma lifetimes in other FRC experiments have typically been limited to values significantly lower than the resistive timescale of the conducting wall, so that time-varying eddy currents did not pose any practical problems and have not been receiving much attention.
One related technique to prevent the excitation of translation-induced eddy currents is the use of insulating axial “gaps” in the vessel to prevent the excitation of axisymmetric eddy currents. The drawback of this method is that it requires structural changes to the conducting vessel, and that eddy currents are not suppressed but axisymmetric currents are transformed into 3-D currents. This thus aggravates the detrimental effects from 3-D fields and also makes the wall unsuitable for passive stabilization of axisymmetric plasma instabilities.
Three dimensional error fields are often corrected by error field correction coils that are themselves not axisymmetric. In the best case, such coils can eliminate as many harmonics as there are coils, but they tend to introduce new errors in the remaining harmonics and need to be able to follow any time-variation of the error fields during the experiment.
Therefore, it is desirable to provide systems and methods that facilitate the reduction or elimination of undesirable eddy currents.